In recent years more and more resources are used towards providing environmentally friendly solutions in many fields of technology, in particular automobile technology. Electric vehicles, such as e.g. hybrid electric vehicles, fuel cell powered vehicles and battery powered vehicles, are rapidly increasing in popularity due to immense advancements being made in propulsion range, power and reliability of the vehicle, so to reach the long term goal of reducing crude oil consumption and emission of harmful pollutants and green-house gas in the world.
A conventional electric motor system consists of a power source, a rectifier with a filter capacitor in case of an AC-feed, or just a filter capacitor in case of a DC-feed and an inverter (motor control circuit). Pulse Width Modulation (PWM) techniques have long been used to improve performance and reliability of power conversion devices and are often used to generate alternating current to the motor in electric vehicles. PWM schemes are used to adjust the amplitude and frequency of the fundamental component of the inverter output and while doing so current is momentarily fed to the motor via the inverter circuit, however, even though that the inductance present in the winding(s) of the motor slows down the rush of current to the motor, there will still be a commutation of inductive current in the motor to momentary current from the feeding circuit, which will result in large ripple AC components over the DC bus.
The conventional way to handle the, often harmful, voltage ripple over the DC bus has been to introduce very large capacitors configured to absorb the DC bus ripple. However, currently available capacitors with the required capacitance are associated with many problems such as cost, size and reliability. Often electrolytic capacitors must be used which increase weight/size of the system, severely reduce the overall life-time of the system and also exhibit poor performance under the ambient conditions present in automotive applications resulting in a need for complex and often expensive adaptations of the whole system.
There is therefore a need for an improved method and system for electric motor control, in particular in automotive applications, to meet the cost, size and lifetime requirements for electric vehicles. Even though the above discussion is focused on electric vehicles, similar situations and problems are encountered in many other types of rotary electric machine applications.